1. Field of the Invention
The present invention relates to a device and a method for driving a cathode discharge tube that is used as a light source for a liquid crystal display, display panel and the like.
2. Description of the Related Art
In recent years, for back-lights in liquid crystal displays of notebook computers and the like, there have been used cold cathode fluorescent tubes and hot cathode fluorescent tubes, which consume a comparatively small amount of electric power and have high luminous efficacy.
So far, in a cathode discharge lighting device that lights these cathode discharge tubes, a DC voltage is converted into an AC voltage by a DC/AC inverter circuit, and then using the AC voltage the cold or hot cathode discharge tube is lighted. The discharge starting voltage for a cold cathode discharge tube is higher than that for a hot cathode discharge tube. Also, the discharge starting voltage becomes higher, as the length of a cold cathode discharge tube becomes greater.
FIG. 16 shows prior circuitry of a cold cathode discharge device that lights a cold cathode discharge tube. As shown by FIG. 16, the cold cathode discharge device has an inverter circuit 311. The inverter circuit 311 comprises switching elements 304a and 304b such as transistors and a step-up transformer 302 that transforms the input voltage into a high voltage. An AC voltage is generated from a DC voltage output from a DC power supply 307 by alternately switching the switching elements 304a and 304b. This AC voltage is transformed to a higher voltage by the step-up transformer 302 and supplied to a cold cathode discharge tube 210.
The operation at the start of lighting is described with reference to FIG. 17. FIG. 17A shows the envelope of the voltage (tube voltage) across the cold cathode discharge tube 210, while FIG. 17B shows the envelope of the current (tube current) flowing through the cold cathode discharge tube 210. In order to light the cold cathode discharge tube 210, a high AC voltage generated on the secondary side of the step-up transformer 302 is applied to the cold cathode discharge tube 210. At this time, before the cold cathode discharge tube 210 is lighted, the voltage across the cold cathode discharge tube 210 rises (see FIG. 17A), but current does not flow because there is almost no load (FIG. 17B). After that, when the voltage across the cold cathode discharge tube 210 further rises and reaches the lighting start voltage, the current suddenly starts to flow while the voltage starts to fall. Subsequently the cold cathode discharge tube 210 has negative resistance, so that the tube voltage falls, and the tube current rises to become a setting current (a predetermined current necessary to maintain the lighting). In the case of cold cathode discharge tube 210, in order to limit the pouring current, a current control element 301 such as a capacitor is connected in serial to cold cathode discharge tube 210. In short, at the start of lighting cold cathode discharge tube 210, an applied voltage higher than the voltage necessary for maintaining the lighting is required by a large margin. Further, the lighting maintenance voltage and lighting start voltage tend to become higher as cold cathode discharge tube 210 becomes longer.
In general, as a method of lowering this discharge starting voltage in a cathode discharge device, there is a method of lowering the discharge starting voltage by grounding a near-by conductor at the perimeter of the cold cathode discharge tube (or a hot cathode discharge tube).
In the method of grounding a nearby-conductor at the perimeter of a cathode discharge tube to lower the discharge starting voltage, a potential difference occurs between the electrode to which a high voltage is input and the near-by conductor, so that an effect of lowering the discharge starting voltage is obtained by a discharge prompting effect. However, in a cathode discharge lighting device, the other cathode and the near-by conductor are both grounded, so that there occurs no potential difference between them. Therefore, in the discharge device shown as a prior art, glow discharge starting from Townsend discharge reaches whole-tube discharge from the high-voltage electrode toward the GND electrode of the cathode discharge tube. In this way, in the prior discharge device, the discharge prompting effect is obtained only at the high-voltage electrode and not at the other electrode, so that the effect is not sufficient for a method of lowering the discharge starting voltage.
Further, as a method for solving the above problem, there is a method disclosed in the Japanese Laid-open Patent Publication No. 8-31588. In the method proposed by the Publication No. 8-31588, a near-by conductor is connected to the middle potential point of the high AC voltage to make the potential of the nearby conductor the middle potential, and thus Townsend discharge is induced from both electrodes to lower the discharge starting voltage. However, in this method, the sustaining voltage for lighting becomes higher as the cold cathode discharge tube becomes longer, so that a leak current is generated by a floating capacity between the nearby conductor and the cold cathode discharge tube. As a result, there are such problems as the lowering of luminance and the enlarging of the discharge device due to reactive power. Also, there is another problem that it is difficult to detect a current flowing through the discharge tube.
The present invention is made to solve the above problems. The object of the present invention is thus to provide a device and a method for lighting a cold cathode discharge tube which can lower the discharge starting voltage by a simple method without degrading the characteristics of the lighting device for a cathode discharge tube even if the cathode discharge tube becomes longer.
In a first aspect of the invention, an apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the AC voltage applied to the cathode discharge tube is raised at a speed slower than a rise speed of the cathode discharge tube.
In a second aspect of the invention, a driving apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the cathode discharge tube is half-lighted by the AC voltage, and subsequently the AC voltage is raised at a speed slower than a rise speed of the cathode discharge tube.
In a third aspect of the invention, a driving apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The apparatus comprises a voltage application section which outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller which controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the cathode discharge tube is half-lighted by the AC voltage, the state of half-lighting is maintained for a predetermined period, and then the AC voltage is raised to a voltage level at which the cathode discharge tube starts discharging.
In the above driving apparatus, the voltage controller may vary the AC voltage stepwise during the sate of half-lighting. Also, in the case where adjustment of light is performed at the time of starting to light the cathode discharge tube by repeating lighting and putting-out, the voltage controller may make the lighting time during the light adjustment shorter than the first lighting time at the start of the lighting.
In a fourth aspect of the invention, a method of driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The method comprises outputting an AC voltage to be applied to the cathode discharge tube, and controlling the output AC voltage. The controlling includes controlling the output AC voltage so that the AC voltage applied to the cathode discharge tube rises at a speed slower than a rise speed of the cathode discharge tube, in order to light the cathode discharge tube.
In a fifth aspect of the invention, a method of driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The method comprises outputting an AC voltage to be applied to the cathode discharge tube, and controlling the output AC voltage. The controlling includes controlling the output AC voltage so that the cathode discharge tube is half-lighted by the AC voltage, and subsequently the AC voltage rises at a speed slower than a rise speed of the cathode discharge tube.
In a sixth aspect of the invention, a method of driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided. The method comprises outputting an AC voltage to be applied to the cathode discharge tube, and controlling the output AC voltage. In order to light the cathode discharge tube, the controlling includes controlling the output AC voltage so that the cathode discharge tube is half-lighted by the AC voltage, the state of half-lighting is maintained for a predetermined period, and then the AC voltage rises to a voltage level at which the cathode discharge tube starts discharging.
In a seventh aspect of the invention, a driving apparatus for driving a cathode discharge tube by applying an AC voltage to the cathode discharge tube is provided.
The apparatus comprises a voltage application section that outputs an AC voltage to be applied to the cathode discharge tube, and a voltage controller that controls the output of the voltage application section. In order to light the cathode discharge tube, the voltage controller controls the output of the voltage application section so that the AC voltage applied to the cathode discharge tube is raised slowly and thereby a protrusion in voltage change does not appear at a moment of lighting of the cathode discharge tube.